1. Field of the Invention
The present invention relates to a CRT display unit. More specifically, the present invention relates to a CRT display unit having a function for automatically adjusting the position and vertical/horizontal sizes of a picture display area to a predetermined position and size in accordance with a plurality of different kinds of video signals with a different timing specification such as an input start timing, input end timing and the like, using the horizontal or vertical synchronization signal of a video signal for a single line or single picture as a reference point.
2. Description of the Related Art
The position and size in the vertical/horizontal directions of a picture display area displayed on the tube surface of a CRT display unit are determined based on a timing specification such as the cycle of the vertical/horizontal synchronization signals contained in a video signal input to the CRT display unit, input start timing, input end timing of the video signal to the synchronization signals, and the like.
More specifically, the size and position of a raster scanning area as an area in which an electron beam is scanned in accordance with the cycle of the horizontal/vertical synchronization signals are determined. A picture display area in which an effective picture is displayed is set to an area smaller than the raster display area and the horizontal size (horizontal width) of the picture display area is determined by a time width from the input start timing to input end timing of a video signal V.sub.L for a single line. The input start timing of the video signal V.sub.L corresponds to the position where the display of a picture for a single line is started. The input end timing of the video signal V.sub.L corresponds to the position where the display of the picture for the single line is ended. Therefore, the time width from the input start timing to the input end timing of the video signal V.sub.L corresponds to the display period Td.sub.H of the picture for the single line. Further, the horizontal position of the picture display area to the raster scanning area is determined by the relative positional relationship on a time axis of the display period Td.sub.H for a single line to the cycle T.sub.1H of the horizontal display signal.
On the other hand, the vertical size (vertical width) of the picture display area is determined by the time width (display period Td.sub.V) from the input start timing (display start position) to the input end timing (display end position) of a video signal V.sub.S for a single picture. The vertical position of the picture display area to the raster scanning area is determined by the relative positional relationship on a time axis between the vertical synchronization signal and the display period Td.sub.V for a single picture.
Usually, the deflection circuit and the like of a CRT display unit are designed in accordance with the predetermined reference timing specification of a video signal so that the horizontal/vertical positions and size of a picture display area has a predetermined position and size.
Incidentally, CRT display units used in recent computer systems are required to display a plurality of pictures each having a different dissolution (for example, 640 dots.times.480 lines, 1280 dots.times.1024 lines, etc.) by changing them. This requirement results from the fact that personal computers having a higher processing speed are made at a lower cost and thus systems with a high resolution can be obtained at a low cost and further application software in use covering various fields, from business to engineering, has an improved function and wide variety.
Further, work stations, CAD systems and the like increasingly require realization of so-called multi-scanning type CRT display units by which the size and position of a picture display area can be automatically adjusted in accordance with a timing specification of various video signals to support various available systems.
To satisfy these requirements, there is conventionally proposed a CRT display unit wherein raster scanning start/end positions are detected from the horizontal/vertical synchronization signals of an input video signal and the number of horizontal dots are determined by an arithmetic operation from preset display parameters such as the aspect ratio of a picture display area, display period ratio and the like to automatically adjust a display size (Japanese Patent Unexamined Publication No. Sho 62-91992).
Nevertheless, since the aforesaid conventional CRT display unit pays attention to the raster scanning start/end positions, a problem arises in that the size and position of the picture display area cannot be automatically adjusted unless a display period to a scanning period is preset in an arithmetic operation block.